1. Field of the Invention
This invention relates to an etchant gas and to a method for etching transistor gates. More specifically, the present invention provides an improved etchant gas and a method for selectively etching with the improved etchant gas a tungsten silicide layer and a polysilicon layer over a gate oxide layer to form a semiconductor structure having straight wall, perpendicular profiles with low microloading and excellent profile control.
2. Description of the Prior Art
During state-of-the-art semiconductor processing of semiconductor devices, many devices are made in a single substrate. These devices are connected to each other by means of conductive lines. However, since these conductive lines can introduce unwanted electric signals in the semiconductor substrate during operation of the devices, the devices must be separated from each other by some means of isolation. The usual means of isolation is to etch trenches between the devices that can be filled with a dielectric material, such as silicon oxide.
Openings in silicon layers, such as tungsten silicide (WSi.sub.x) and polysilicon, are also etched for various semiconductor processes. For example, trenches are formed in polysilicon layers that will be filled with silicon oxide. The sidewalls of the trenches must be as straight as possible so that no voids remain after filling the trench. The bottom of the trench must have a smooth line between the sidewall and the bottom. Stated alternatively, there should be no microtrenching between the sidewall and the bottom of the trench. Further, it is desirable to have a high etch rate consistent with the above criteria.
Various etchants are known for silicon, such as halogen and halogen-containing etchants. HBr is known to form deep straight-walled openings in silicon and polysilicon. However, the etch rate is low and HBr is corrosive to expensive equipment. Chlorine alone is unsatisfactory because the etch is isotropic. Oxygen can be added to chlorine for etching trenches, which provides a higher etch rate, but the sidewalls are not straight, resulting in a bowed profile. A mixture of helium and oxygen (e.g. 70% by vol. He and 30% by vol. O.sub.2) has been used to improve the etch rate, but the mixture causes undesirable microtrenching in a gate oxide layer.
Therefore, what is needed and what has been invented is an improved etchant gas that is highly anisotropic, that has a high etch rate, and that provides trenches with straight walls. What is further needed and what has been invented is a method for etching conductive layers supported by a gate oxide (SiO.sub.2) layer to obtain smooth sidewalls without any associated microtrenching of the gate oxide layer.